Portable desk



FIG. 1 is a front perspective view of a portable desk showing my new design;

FIG. 2 is a rear perspective view thereof;

FIG. 3 is a rear perspective view with the extendable support arm in position of use; the broken line showing of a chair is for illustrative purposes only and forms no part of the claimed design; and

FIG. 4 is a side elevational view thereof. 

The ornamental design for a portable desk, as shown and described. 